Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising.
FIG. 1 illustrates a powertrain 10 with a transmission 12 incorporating a park-by-wire system. The flow of mechanical power is illustrated by solid lines, while dashed lines indicate the flow of information signals. Power is generated by engine 14 and conveyed to a transmission input shaft 16. A torque converter 18 and gearbox 20 modify the speed and torque at which the power is delivered to match vehicle requirements while permitting engine 14 to run at a suitable crankshaft speed. Other types of transmissions may utilize other types of ratio manipulation devices. Driveshaft 22 transfers power from transmission 12 to differential 24. Differential 24 distributes the power between drive wheels 26 and 28 while allowing slight speed differences such as when turning a corner. Some transmissions, such as front wheel drive transaxles, may include the differential in the same housing with the gearbox and torque converter. In such transmissions, power transfer to the differential may utilize gears or chains as opposed to a driveshaft. In some vehicles, a transfer case may be interposed between the transmission and differential to transfer some power to additional wheels.
Torque converter 18 includes an impeller driven by the input shaft 16 which hydro-dynamically drives a turbine. Torque is transferred from the impeller to the turbine whenever the turbine is rotating slower than the impeller, including when the turbine is stationary. Torque converter 18 also includes a stator such that the torque exerted on the turbine may be a multiple of the torque exerted by the impeller on the input shaft. The torque converter may also include an actively controlled lock-up clutch to allow efficient transfer of torque without any speed difference between the impeller and the turbine. The gearbox may be a discrete ratio gearbox that selects is capable of establishing various power flow paths with various speed ratios by selectively engaging various combinations of clutches. Alternatively, the gearbox may include a continuously variable ratio mechanism.
Transmission controller 30 adjusts the state of transmission 12 based on various inputs, including vehicle speed measurements, driver torque demand as indicated by accelerator pedal position, and shift selector 32. The driver uses the shift selector to indicate the desire to move forwards (D) or backwards (R) for example. Controller 30 may adjust the state of the transmission by sending signals to valve body 34. In response to these signals, valve body 34 adjusts the pressure in hydraulic circuits to engage particular clutches, such as clutches within gearbox 20 and the torque converter lock-up clutch.
Park mechanism 36 is engaged in response to driver selection of park (P) via shift selector 32 in order to prevent vehicle movement when the vehicle is unoccupied. The park mechanism must be designed such that, once engaged, the system stays engaged without any vehicle power. In some vehicles, the park mechanism is mechanically linked to the shift selector 32. However, in a park-by-wire system, transmission controller 30 engages and disengages park mechanism 36 in response to an electrical signal from the shift selector 32. Controller 30 may control the park mechanism indirectly by sending electrical signals to valve body 34 that result in hydraulic or mechanical signals to the park mechanism 36.